Lockers used to tension a Slackline safe to climb with?

As the title implies, I'm wondering about the safety of using some BD locking carabiners that I've used to tension a slackline to climb with, primarily for building anchors. I purchased these carabiners before my climbing days to build slacklines between trees the cheapo way, using just a length of webbing, and a Z-pully at one end with the locking biners.

So, they haven't taken any falls or look structurally damaged â I'm just not really sure how safe they might be given they held a tensioned force of however much while used for slacklining. I'd guess around 4-5KN of force.

In hindsight, I HAVE used these same locking carabiners for the occasional climbing a few years later, completely forgetting about using them for slacklining first. Are these safe to continue using or should I just bite the bullet and replace them?

Pardon even needing to post this, I know when in doubt you should always opt for new gear you can trust, but it's a slippery slope of always questioning your gear. As a college student dropping however much on new biners / gear is tough to justify sometimes.

edepow wrote:Hey everyone, As the title implies, I'm wondering about the safety of using some BD locking carabiners that I've used to tension a slackline to climb with, primarily for building anchors. I purchased these carabiners before my climbing days to build slacklines between trees the cheapo way, using just a length of webbing, and a Z-pully at one end with the locking biners. So, they haven't taken any falls or look structurally damaged â I'm just not really sure how safe they might be given they held a tensioned force of however much while used for slacklining. I'd guess around 4-5KN of force. In hindsight, I HAVE used these same locking carabiners for the occasional climbing a few years later, completely forgetting about using them for slacklining first. Are these safe to continue using or should I just bite the bullet and replace them? Pardon even needing to post this, I know when in doubt you should always opt for new gear you can trust, but it's a slippery slope of always questioning your gear. As a college student dropping however much on new biners / gear is tough to justify sometimes. Thanks.

You estimated they experienced 4-5kN....and they're rated for what, 22kN? More? What do you think? They're fine. I wouldn't give it a second thought.

I keep slacklining gear separate from climbing gear. I'm generally firmly in the camp of, "if it passes visual inspection it's fine" for hard gear and even well stored soft goods. But slacklining is the exception for me since I've seen several carabiners pulled in half in slackline setups (one in person, a few more online). True the gate probably ended up open or something like that, but regardless. In my mind it's not that hard to imagine the high, cyclic loads of someone bouncing on a tight slackline might have some impact on the biner.

This tension is being supported by a carabiner, which is serving as a little 2:1 pulley with 70% efficiency due to friction, so I'm thinking that the carabiner supports 1.7x the line tension, leading to a total force of 15kN.

I think I'm intentionally being conservative here (1ft sag over 20ft is a pretty tight slackline), but that's still a bigger number than I initially expected. Comparing to carabiner strength as reported by some MIT presentation I found online, some arbitrary climbing carabiners survived 1000-3000 cycles at that load before failure.

So the next logical question is have you bounced more or less than 1000 times on your slackline? :)

Brandon Seerup wrote:Just for fun I tried to calculate some numbers. Feel free to critique. 20ft slackline deflecting 1ft in the middle, with a 200lb guy, generates 1000 lbf tension in the line. Let's say our guy bounces and generates 2x his weight in force, so 2000 lbf tension --> ~9kN. This tension is being supported by a carabiner, which is serving as a little 2:1 pulley with 70% efficiency due to friction, so I'm thinking that the carabiner supports 1.7x the line tension, leading to a total force of 15kN. I think I'm intentionally being conservative here (1ft sag over 20ft is a pretty tight slackline), but that's still a bigger number than I initially expected. Comparing to carabiner strength as reported by some MIT presentation I found online, some arbitrary climbing carabiners survived 1000-3000 cycles at that load before failure. So the next logical question is have you bounced more or less than 1000 times on your slackline? :)

Question on the math: agreed that the bouncing force is probably low balled. however, the webbing is dynamic, which would eat force the further from the biner one got. Also this assumes that the biner takes full force, when that will be split between the two anchor points. Im sure thwt I'm missing a few other variables, land speed of a swallow and whatnot, but I think the numbers get lower. We've set rather long lines on simple friction pulleys and the wear and tear on the webbing becomes significant long before the metal.

Still I separate my gear, if only because it makes it harder to forget pieces.

bking7 wrote: Question on the math: agreed that the bouncing force is probably low balled. however, the webbing is dynamic, which would eat force the further from the biner one got. Also this assumes that the biner takes full force, when that will be split between the two anchor points.

Tension in the line is tension in the line. So no, the force is not split between two anchor points.

Only thing you need to worry about is when you climb with a biner it can get damage on it that could cut a slackline later. I can't think of any way a slackline biner would have any damage that would hurt it's use for climbing.

I guess if you are someone doing multi flips / jumps on it repeatedly every day for a year it may be a different issue of general wear... but for the average person who sets it up and does some walking once or twice a week probably not much of a worry.

The primary concern is fatigue, which is a very real problem with aluminum. I have seen aluminum and steel components used in slacklining fail from fatigue. Just a few days ago, a slacklining company posted a photo of a Fixe 40kN hanger that failed from long term use in slacklining where the slackline was rigged 24/7 in a gym.

Anyway, if you're just setting up a loose line for a day, sure, using some biners off your rack is fine. If you're going to be using your slacklining rig regularly, I would strongly recommend getting dedicated biners. You can get some lockers for like ~$7-8.

Aluminum carabiners designed for climbing/mountaineering are not appropriate for the tensioning of slackline systems. Aluminum fatigues; the vibration in the slack line can easily provide 10s of thousands of loading cycles to induce cracks to propagate.

ViperScale wrote:None of those you posted are biners though. I know the gym I went to had a slackline up 24/7 with biners in it and even after years and tons of people on it never broke a single one.

Nope, they're not, but aluminum is much more susceptible to fatigue than the stainless steel used in the hanger, so it certainly could be a problem. In the end, it really depends on the loads that the line sees. For a loose line the biners may be able to take millions of cycles, but for a highly tensioned line, a biner may fatigue after only a few hundred cycles. As with everything in climbing/slacklining, it depends.

bking7 wrote:however, the webbing is dynamic, which would eat force the further from the biner one got.

BIG misunderstanding here. The dynamic nature of the rope does not magically "eat" force. Force in one end is going to come out the other end (neglecting slight losses due to internal rope friction or such).

Dynamic rope is used to reduce peak loading but spreading the energy over a longer period of time, but it doesn't reduce the force transferred from one end to the other.

bking7 wrote:Question on the math: agreed that the bouncing force is probably low balled. however, the webbing is dynamic, which would eat force the further from the biner one got. Also this assumes that the biner takes full force, when that will be split between the two anchor points. Im sure thwt I'm missing a few other variables, land speed of a swallow and whatnot, but I think the numbers get lower. We've set rather long lines on simple friction pulleys and the wear and tear on the webbing becomes significant long before the metal.

As someone indicated, tension is tension, and is equal at all points in the webbing (theoretically). Where stretch will help you, and what I didn't account for, is that the slackline will get longer with increased load and help reduce the sharp angles that multiply force. Hard to say how much this helps.

dave custer wrote:Aluminum carabiners designed for climbing/mountaineering are not appropriate for the tensioning of slackline systems. Aluminum fatigues; the vibration in the slack line can easily provide 10s of thousands of loading cycles to induce cracks to propagate.

Good point. High cycle fatigue can easily result from vibrations in a system like this, and Aluminum has no fatigue limit. From wikipedia: "Other structural metals such as aluminium and copper, do not have a distinct limit and will eventually fail even from small stress amplitudes".

Old lady H wrote:I purchased BD carabineers not too long ago (rocloc and positron) and the sheet is pretty generic. All 5 styles shown have an X and you are gonna die on the slacklining pic. Just saying. ;-)